In the reductive thermal coal treatment of metallic minerals containing metal oxides, the use of rotary hearth furnaces is known. Such furnaces generally consist of an annular chamber having the bottom wall, the so-called rotary hearth, rotating relative to the remainder of the chamber, a plurality of burners being disposed on the side walls and on the top wall of the furnace. Pellets consisting of the mineral containing the metal oxide which is to be treated mixed with coal are usually introduced into the furnace and deposited on the rotary hearth, in order to favor the evolution of CO, which is the effective reducing agent. The heat supplied by the burners allows the heterogeneous coal/mineral mixture to reach the right temperature for the reduction reaction.
The furnaces constructed in this way have nevertheless disadvantages, both from the point of view of the economics of the operation and from the point of view of the environmental impact. On the one hand, the reaction of reducing the metal oxides with C, or rather CO, is in fact endothermic and therefore attains the best efficiency levels at elevated temperatures; Therefore, a good process yield involves a significant energy consumption which inevitably increases the operating costs.
On the other hand, the atmosphere in the interior of the furnace chamber is rich in CO and produces a discharge gas with high pollution potential; consequently, the environmental impact of the thermal coal treatment in question is rather great.
In EP-A-0 508 166 is described a rotary hearth furnace in which the waste gases are transferred from a first zone of the chamber downstream of the feeder means and upstream of the gas extraction means, with respect to the direction of rotation of the hearth, into a second zone upstream of the material discharge means and downstream of the gas extraction means, through an air-gas burner. The first zone is separated from the other parts of the chamber by a curtain that only allows the passage of the mineral on the surface of the rotary hearth. However, the flow rate of gases from the first zone to the second zone can't be regulated. Morever, the combustion of such gases in the second zone is not carried out as a diffused combustion, and this can affect the efficiency of the process.